Modulation transfer function of the imaging probe using an 8.8 mm-long and 125 µm-thick graded- index short multimode fiber

In vivo imaging of tissues offers vast opportunities in biological and other medical fields. Reliable and robust endoscopic probing as well as imaging catheters are in high demand in several biomedical optic applications. In a bid to minimize trauma arising from insertion of needles in applications such as delicate organs imaging, using very thin needles is preferable.

Probes implementation can be achieved through forward or side imaging. In the latter, deep skeletal muscles are imaged with the help of thin optical needle probe with an outer diameter of approximately 310µm. A needle endoscope with a bundle of fiber has been applied in the forward-imaging. The endoscope is simply a reflectance-mode fiber microscope. However, most of these probes are large for minimally invasive imaging.

In a recent research published in Optics Communications scientists led by Professor Manabu Sato at Yamagata University in Japan, determined the modulation transfer function by applying short multimode fiber with a length of 8.8 mm and 125 µm diameter. The authors proposed an optical model and a phase screen to explain blurs. They also compared, in their work, dependence of the contrast on wavelengths and simulation and experimental results.

The authors adopted an experimental setup that would help them assess the graded-index short multimode fiber, which was designed for optical communications (Fujikura Ltd., Future Guide-MM50), based on an optical microscope. The authors imaged the fiber together with a sample using a camera and a Laser displace gauge. They obtained the distance between the fiber and the specimen as imaging conditions by calibrating the images obtained.

The authors determined the dependence of contrasts on wavelengths of 540nm, 632nm, 730nm, and 852nm using a test pattern of 228lp/mm (period of 4.38µm). They noted that the black and white were inverted between the reflection and the transmission images. For the reflection images, they recorded a high signal intensity and low contrast at a wavelength of 540 nm. However, at a wavelength of 852 nm, both signal intensity and the contrast were low. The authors concluded that the images at 852 nm wavelength degraded because of low quantum efficiency of the camera and low optical power of Halogen lamp.

The authors observed that the contrast decreased with the increase of the spatial frequency. Without the random phase screens in the short wavelength zone, the authors noticed an improvement in the dependence of contrast. However, for zones with high frequency, they recorded a drop in the contrasts as the spatial frequencies increased. With the random phase screens, they observed a drop in the contrast as the wavelengths became shorter. However, in the long wavelength regions, the contrasts were approximately similar to those without the random phase screens.

This study has, therefore, presented an optical model based on short multimode fiber that the authors managed to fabricate. The study also focused on using random phase screens to characterize the degradation of the imaging attributes and measuring the modulation transfer function. The authors, therefore, proposed an 8.8mm short multimode fiber optical model. The outcomes of the study are helpful in designing imaging optics and estimate their performances. This form as an imaging probe is prime because of an ultra-thinness for less invasion, high stability and reliability due to a simple structure and low cost. This might open new applications in fields of physiology and clinical medicine.